Cultural: The nature of political alliances in Mesoamerica made them forever fragile. Even under the widespread and powerful Aztec empire there were frequent rebellions by a single city-state, or more seriously, by a new confederation of city-states (Ross Hassig. Aztec Warfare. University of Oklahoma Press, 1988, pp. 22-26).
These rebellions were put down only by extreme military force. Here we have the very same situation. Three city-states have elected to rebel against the over king to whom they have been allied. As with all other such rebellions of separation, it will succeed unless it is quashed through military might. As we will see, that will not happen in this case.